This application claims the benefit of the filing date of U.S. Provisional Patent Application No. 60/851,646 filed Oct. 13, 2006, the disclosure of which is incorporated herein by reference.
The following application includes the article Gao and Honer, “Magnetic Shielding for Stacked MRAM”, Semiconductor Packaging, Dec. 1, 2006 which is incorporated by reference herein.
Magnetoresistive random access memory (“MRAM”) is a form of computer memory which uses magnetic polarity to store data. The use of magnetic polarity is advantageous because it results in a non-volatile form of memory, which means that it does not necessarily require an electrical current to retain the data stored therein. A typical MRAM structure uses many individual cells, each formed of two layers of ferromagnetic material separated by a thin layer of an insulating material. Each cell is capable of storing one bit of memory, which is carried out by imparting either a positive or negative polarity to one of the two ferromagnetic layers, the other layer having a fixed polarity. Essentially, when the two plates have the same polarity, the cell represents either a one or a zero, according to typical methods for storing memory, a cell with plates of the opposite polarity representing the opposite.
The polarity of a cell is changed by using a series of bit and word traces. This is done by arranging the individual cells in an array of rows and columns lying in, for example, the X- and Y-directions in a plane. The bit traces and word traces overlie, respectively, the rows and columns of cells. In this arrangement, one bit line and one word line intersect at the location of each of the cells. In order to “write” data to a cell, currents are passed through the corresponding bit and word lines, inducing a magnetic field at the location of the cell. The writable ferromagnetic plate matches this magnetic field. When the induced magnetic field ceases, the writable plate maintains the field imparted on it. This is done for all of the cells in the array in a predetermined sequence, thereby writing data to be retained in the memory.
The tunnel magnetoresistance effect (“TMR”) causes the resistance of the insulator between the plates to change depending on the state of the cell. Generally, the resistance of the insulator is higher when the polarity of the plates is different (or “antiparallel”) than when the plates have the same polarity (parallel). This effect is used in reading data from the memory.
Due to the nature of the MRAM structure, the data stored therein is susceptible to being corrupted or destroyed by stray magnetic fields. Essentially, a stray magnetic field which encounters the memory array can change the polarity of one or more cells, thereby corrupting the memory therein. A strong enough field could conceivably change all of the cells to the same polarity, thereby erasing all of the data stored in the array.